Swelling seals are well from, for example, WO03/008756 and US2007/0056735. These documents disclose seals with an annular form and a cylindrical wall having a radial wall thickness in a non-swollen state and an increased radial wall thickness in an expanded state. The sealing effect takes place by swelling, which transfers the seal from the non-swollen to an expanded state, the increased radial wall thickness providing the sealing effect. The maximum increase in radial wall thickness is determined by an absolute swell. The disadvantage of these swelling seals is that the absolute swell limits the width of an annular space that can be bridged and then sealed. This is caused by the fact that an annular space with a considerable width also requires a seal with a considerable radial wall thickness in a non-swollen state. However, a seal with a considerable radial wall thickness in a non-swollen state may be difficult to be transported to the annular space where it has to perform its sealing function due its thickness in a non-swollen state. In many cases the annular space where the sealing is required is several hundred meters downhole inside the bore hole from a well. A seal with a large diameter in the non-swollen state is not always desirable or practical.
In order to remove this disadvantage, WO2013/095093 discloses a seal sleeve having a bellow-shaped seal wall of swelling material. In an expanded state, the slanted seal wall parts of the bellow that extend radially outwardly not only become thicker due to the swelling but also increase in length in a radial direction, so that the radial width of the annular space that may be closed off can be larger. The radially outward extending wall parts cover a distance multiple times the wall thickness of the seal wall. The bellow shaped seal wall must be rather thin to obtain a good bellow configuration. However, a thin seal wall has a limited strength, which may be disadvantageous for some applications.
Although the seal disclosed in WO2013/095093 may be used to seal annular spaces with a considerable width, due to the slanted wall sections, it still has a considerable thickness in a non-swollen state.
WO2011/020987 discloses a pressure control device for isolating a section of a conduit, the device comprising a support member, a flexible cup member mounted to the support member and a first swellable element. The first swellable element adapted, upon activation by an activation fluid, to urge a first portion of the cup member outwards into engagement with a conduit surface. The first swellable element includes at least one bypass arranged to permit the activation fluid to bypass the swellable element and build up behind a second portion of the cup member. The second cup member portion is adapted to be moved under the action of the fluid pressure into engagement with the conduit surface. The pressure control device known from WO2011/020987 is complicated in that it requires at least three parts, i.e. a support member, a flexible cup member and a swellable element. Additionally, bypasses are required to transport fluid to an annulus downstream of the swellable element so that is contacts a middle section of the flexible cup member. By virtue of fluid pressure build up in the annulus downstream of the swellable element, i.e. not by virtue of swelling, the middle section of the flexible cup is forced into engagement with the wellbore surface. When the fluid pressure is low, the sealing action of the pressure control device will be poor. The complicated structure makes the known pressure control device costly and difficult to manufacture.